1. Field of the Invention
This invention relates generally to the structure of a flat-type brushless DC motor and, more particularly, to such motor in which a side force is continuously applied to the rotor to prevent epicyclic motion of the rotor shaft or axle.
2. Description of the Background
Flat, thin, brushless DC motors, so-called spindle motors, have generally been known for use in various applications and one particular use is for the disc drive section of an electronic still camera. Such brushless DC motors are generally known to include a rotor and a stator, which rotatably supports the rotor.
In such flat, brushless, DC motors the rotor typically includes a dish-shaped rotor yoke with a rotary axle projecting downwardly to the center and a circular chucking magnet affixed to an internal peripheral surface of an annular rib that integrally projects upwardly from an upper surface of the rotor yoke. The chucking magnet holds the magnetic disc for recording. A disc-shaped circular rotor magnet is affixed to a lower surface of the rotor yoke by means of disc-shaped circular auxiliary yoke.
The stator of such brushless DC motor typically includes a disc-shaped stator yoke and a boss formed in the center thereof to which is affixed an oil-containing, cylindrically shaped, metal bearing that is staked into an open center portion of the stator yoke. The oil-filled metal bearing is provided to support the axle of the rotor and at the lowermost end where it is affixed to the boss a disc-shaped thrust bearing is provided as an axial bearing support for the rotor axle. A flexible printed circuit board has formed thereon a six-phase sheet-like coil, which is radially disposed with the boss as a center and is spaced apart from the rotor magnet with a predetermined clearance. The printed circuit board is affixed to the flat surface of the stator yoke that is facing the rotor magnet.
In the operation of such brushless DC motors, when a current flows into the sheet-like coil of the stator yoke thereby magnetizing the sheet-like coil, the coil is attracted to a magnetic pole of the rotor magnet so as to produce a rotary force to drive the motor. Thus, the rotor is rotated in a predetermined direction with the rotor axis as a center. The magnetic disc of the electronic still camera is caused to rotate with the rotation of the rotor by being attracted to the chucking magnet that is mounted on the upper or outer surface of the rotor.
As in all consumer products the cost of manufacturing the product is extremely important and in the brushless motor used in the disc drive section of the electronic still camera the cylindrically shaped, oil-containing metal bearing is employed s a substitute for a much more expensive ball bearing. It is known that in the use of such oil-filled bearings some clearance must be provided between the rotary axle or shaft and the metal bearing and further such clearance is generally emphasized by the difference between expansion coefficients of the bearing material and the material of the axle of the rotor, that is, there is a measurable difference in the temperature response characteristics of the respective two metals. Thus, it is seen that because of this clearance, which may vary in response to temperature, it is impossible to avoid vibrations of the axle relative to the bearing caused by such clearance, unless a side force is continuously applied to the axle to cause it to remain in the same place relative to the bearing. Furthermore, over the life of such brushless DC motor wear will occur between the axle and the bearing caused by friction and, thus, the clearance initially provided between the axle and the bearing will increase. This further provides and opportunity for non-periodic vibrations of the axle which cannot easily be eliminated by realignment and further a generally periodic axle vibration may also easily occur with such increase in clearance between the bearing and axle.
Therefore, in view of such motor vibrations, if a brushless DC motor having vibrations in the rotor axle is used as the rotary drive for a high density magnetic disc, as might be used in an electronic still camera, whose track width is narrow, severe problems occur during recording on such magnetic discs.
One example of a brushless DC motor attempts to prevent vibrations of the rotor axle by providing a uni-directional thrust to the rotor
In that spindle motor, an additional magnet is installed between the axle and the stator magnet so that a side force is continuously applied to the rotor axle in a lateral direction by the magnetic attracting force of the magnet on the axle. In this fashion, the axle is attracted toward the particular side of the bearing on which the additional magnet is located, thereby preventing periodic and/or non-periodic vibrations on the rotor axle.
Nevertheless, as pointed out about because the very reason for using the oil-filled bearing in the first place is to effect a cost saving, it has been found that by providing the additional magnet to overcome the rotor axle vibration problems that this additional magnet requires additional installation time, as well as increasing the number of assembled parts in the motor and increasing labor cost and the construction of the motor becomes unduly complex. Consequently, manufacturng costs of the whole motor are increased. Furthermore, because the spindle motor is intended to be a substantially small-sized motor, there is not sufficient space in a flat, brushless, DC spindle motor to accommodate a strong permanent magnet that is capble of providing a sufficiently strong side pressure to the axle that can meet the requirement for reduced rotor axle vibration.